Python returns first and last item of a sequence exchanged

Question

I need to create a function to slice a sequence in order that first and last item is exchanged and the middle stays in the middle. It needs to be able to handle string/list/tuples. I am having trouble with typeerrors - cant add list + int.

This:

def exchange_first_last(seq):
    """This returns the first and last item of a sequence exchanged"""
    first = seq[-1]
    mid = seq[1:-1]
    last = seq[0]
    return print(first+mid+last)

produces

(5, [2, 3, 4], 1)

But I don't want a list inside a tuple, just one flowing sequence.

(5,2,3,4,1,)

Any hints/suggestions welcome. The idea is to slice properly in order to handle different object types.

What is `seq` value? As I don't understand what you want to achieve. — Sergey Pugach, Dec 20 '18 at 05:45
Extract the fist and last elements as slices too; `first = seq[-1:]` and `last = seq[:1]`. Then they'll have the same type as `seq`, and addition will work fine. — Tim Peters, Dec 20 '18 at 05:47
Why do you return the return value of `print`, which is `None`? — Jan Christoph Terasa, Dec 20 '18 at 06:03

score 4 · Accepted Answer · answered Dec 20 '18 at 05:55

4

Try this:

def exchange_first_last(seq):
    """This returns the first and last item of a sequence exchanged"""
    first = seq[-1:]
    mid = seq[1:-1]
    last = seq[:1]
    return print(first+mid+last)

answered Dec 20 '18 at 05:55

Ayush Kesarwani

518
6
18

1

I just did this myself and it worked, came back here and saw what people said and here you are! Thanks much! – Iansberg Dec 20 '18 at 19:09

score 1 · Answer 2 · answered Dec 20 '18 at 05:44

1

Change your code a little, notice the square brackets:

def exchange_first_last(seq):
    """This returns the first and last item of a sequence exchanged"""
    first = seq[-1]
    mid = seq[1:-1]
    last = seq[0]
    return print([first]+mid+[last])

Note it actually gives you a list, i.e. [5,2,3,4,1], not a tuple (5,2,3,4,1).

answered Dec 20 '18 at 05:44

Quang Hoang

146,074
10
56
74

2

type(seq)([first]+mid+[last]) to make it a tuple when necesarry although I think a if is just much better – Dec 20 '18 at 05:47

score 0 · Answer 3 · answered Dec 20 '18 at 05:48

You can use list.extend():

def exchange_first_last(seq):
    """This returns the first and last item of a sequence exchanged"""
    first = seq[-1]
    mid = seq[1:-1]
    last = seq[0]
    merged = []
    merged.extend(first)
    merged.extend(mid)
    merged.extend(last)
    return merged

score 0 · Answer 4 · answered Dec 20 '18 at 06:16

0

You could just swap the elements

def swap(l):
     temp = l[0]
     l[0] = l[-1]
     l[-1] = temp
     return l

answered Dec 20 '18 at 06:16

dadrake

112
7

This will not work with immutables like strings and tuples. – najeem Dec 20 '18 at 06:19
Didnt realize it was tuples – dadrake Dec 20 '18 at 06:27

Python returns first and last item of a sequence exchanged

4 Answers4